Conceptual project on using the pumping scheme to eliminate acid mine drainage

Expanded Title:

Acid mine drainage (AMD) poses a serious threat to water quality in the Witwatersrand region, owing to extensive, abandoned mine voids that have subsequently flooded. The formation of AMD is a spontaneous reaction that requires a sulphide mineral (primarily pyrite), water and oxygen to regenerate ferric iron in order to produce net acidity. In response to the severity of the current situation, the South African government has undertaken construction and expansion of facilities to lower the water table to a safe level (e.g. where no AMD decants from the mine into the surrounding groundwater or surface water) in the worst affected portion of the basins through pumping and mine water treatment.
We proposed using the same infrastructure that is currently being installed to manage the problem more effectively and sustainably by pumping from a greater depth, of approximately 2 km below surface. It is hypothesised that the anoxic conditions at this depth will effectively preclude the generation of AMD, allowing the pumps to deliver cleaner water to surface, and hence reducing the need for water treatment while maintaining the desired water level.
In this report, the increase in energy required by pumping from a depth of 2 km was calculated relative to the current depth of just below the environmentally critical level. It was determined that the expected energy increase would be in the region of 10%, suggesting that it is feasible to use the existing/planned pumping infrastructure at this depth.
In the second part of this study, we attempted to quantify how pH, temperature and initial availability of oxygen in reaction water affected the formation rate of AMD from Witwatersrand coal waste rock. We observed no statistically significant effects of initial oxygen availability, pH or incubation temperature on the production of sulphate from coal mine waste rock. In particular, decreased availability of oxygen in incubation water did not inhibit production of sulphate. We did however note that dissolved oxygen (DO) in many of the low DO treatment replicates increased, even though the incubation vessels were sealed, suggesting contamination.
Finally, we studied how the aquifer would change by pumping from depth. In both cases, where we added oxygenated or DO-poor water to the surface and pumped from the bottom of the column, we saw improved water quality with time (pH, EC, Fe and sulphate). In the context of this experiment, there is still considerable uncertainty as to whether this is a result of dilution without any formation of AMD or if it was as a result of dilution and no generation due to all insufficient sulphide present to cause ongoing reaction.
The increase in energy required by pumping from a depth of 2 km was calculated relative to the current depth of just below the environmentally critical level. It was determined that the expected energy increase would be in the region of 10%, suggesting that it is feasible to use the existing/planned pumping infrastructure at this depth. In the second part of this study, we attempted to quantify how pH, temperature and initial availability of oxygen in reaction water affected the formation rate of AMD from Witwatersrand coal waste rock. We observed no statistically significant effects of initial oxygen availability, pH or incubation temperature on the production of sulphate from coal mine waste rock. In particular, decreased availability of oxygen in incubation water did not inhibit production of sulphate.